In recent years, a voice service in which a voice signal is IP packetized and the packetized voice signal is transmitted/received via an IP network is widely used in addition to a voice service that uses a telephone network. For example, Japanese Patent Application Laid-Open No. 2007-36813 discloses a voice service system using an SIP (Session Initiation Protocol). The system includes a plurality of subscriber terminals which are provided a voice service, a plurality of subscriber accommodating apparatuses which are provided at an access edge of an IP network and accommodate a subscriber terminal for each predetermined area unit, and at least one SIP server. In the system, when a source subscriber terminal communicates with a destination subscriber terminal, an SIP session connection request is transmitted from the source subscriber terminal to the SIP server via the subscriber accommodating apparatus. The SIP server which receives the connection request transmits the connection request to the destination subscriber terminal. Thereby, the SIP session is established between the source subscriber terminal and the destination subscriber terminal.
However, the subscriber accommodating apparatus in this system transfers all the SIP session connection requests transmitted from the subscriber terminals which are accommodated in the subscriber accommodating apparatus to the SIP server. Accordingly, in this system, there are concerns that the SIP server becomes congested when SIP session requests that exceed the processing capability of the SIP server are transferred to the SIP server. When the SIP server is in a congested state, problems occur in which the call connection process time becomes long, the SIP session connection request itself is disposed or the like.
For example, Japanese Patent Application Laid-Open No. 2007-110411 discloses the technology as one of technologies for solving the above-mentioned problem. In the technology, when many SIP session connection requests that exceed the processing capacity of the SIP server are generated, an access device transfers the session that cannot be processed to another device from the SIP server. Another device returns a voice response indicating that the connection to the session cannot be made.
For example, Japanese Patent Application Laid-Open No. 2005-269217 discloses the technology in which when the session connection request from a subscriber terminal exceeds a memory area for a call control that is assigned as a system in the SIP server, a part of the calls under communication is saved in an external storage device.
For example, Japanese Patent Application Laid-Open No. 2005-244964 discloses the security gateway which checks a number of simultaneous connection sessions in order to restrict that the clients having the same user ID perform a session setting simultaneously. The security gateway performs a session setting process to the session setting request when the number of simultaneous connection sessions does not exceed a predetermined value and performs an unauthorized connection prevention process when the number of simultaneous connection sessions exceeds the predetermined value. Here, the unauthorized connection prevention process is the process for preventing the session setting from being performed two or more than two times by the same user equipment. When it is determined that an old session is dead, the security gateway eliminates the old session and sets a new session. On the other hand, when it is determined that the old session is valid, the security gateway refuses the new session setting.